Tube drawing



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VACUUM 3,190,739 TUBE DRAWING Richmond W. Wilson, Corning, N.Y., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York Filed Nov. 15, 1961, Ser. No. 152,498 7 Claims. (Cl. 65-85) The present invention relates to improvements in methods of and apparatus for the continuous drawing of glass tubing from a molten supply body thereof. Conventionally, tubing is generally drawn from such a body either downwardly as illustrated for example in Vello Patent No. 2,009,793, or upwardly, as illustrated for example in Woods Patent No. 2,085,245, and the invention is applicable to either general type of system. a

It hasbeen the usual practice in continuous tube drawing operations to direct a stream of air through the tubing being drawn and to regulate the size and wall thickness of the tubing so drawn by suitable control of the viscosity of the molten body, by the air pressure maintained within such tubing and/ or by variation of the rate of draw.

Difliculties in tube size control in accordance with past practices become more pronounced with each increase in diameter of tubing to be drawn, since, as will be appreciated, the volume of air required to be passed through the tubing to maintain the desired diameter must be correspondingly increased. When a large volume of air is passed through the tubing the tube wall forming elements tend to become excessively cooled and interfere with the free movement of the tubing thereover, and often makes extensive auxiliary heating of these elements necessary.

According to the invention the tubing is drawn through a die or mold of a cross sectional configuration in accordance with the desired cross sectional configuration of tubing to be drawn and of an internal dimension or dimensions calculated to impart to the drawn tubing the desired outside dimension or dimensions. The molten tubing issuing from its source of production in its region of entry into the die is of a smaller outer dimension or D. than the inner dimension or ID. of the die and is passed through an aperture of a plate covering the entrance end of the die closely surrounding such molten tubing. Preferably such plate is slightly spaced from the entrance end of the die and provides an annular entrance over. the die end from a vacuum chamber surrounding it. It may be made of graphite, or of a metal preferably treated to prevent danger of the glass sticking thereto. The height of such space compared to the transverse distance between the aperture bordering portion of the plate and die bore bordering wall is made small to prevent any possibility of the tubing being expanded thereinto and thus interfere with the drawing operation. A similar vacuum chamber is provided about the exit end of the die and embodies an end plate whose aperture corresponds in size and cross section to that of the die bore. Additionally the outer end of the chamber at the exit end of the die is provided with an iris or with an axially split plate having an aperture of substantially the diameter of conventionally drawn tubing to be initially passed through the die and serves as a temporary seal thereabout. Such iris or plate is later laterally displaced by the expanded tubing as it passes through the die. Facilities are also provided around the die along its length to regulate its temperature.

While molten tubing is being drawn through the die, by a conventional pulling machine, vacuum is applied to both vacuum chambers to withdraw air from the entrance and exit ends of the die. The degree of vacuum created about the molten stream occupying the entrance end of the die is sufiicient to overcome the viscous resistance of the glass and, accordingly, the tubing expands out into the upper regions of the die until it comes into direct contact therewith. As it does so it is rapidly cooled by such contact with the relatively cool die wall and thermally contracts slightly until the viscosity of the glass is too great for the vacuum to maintain the so-chilled tubing in direct contact with the die. However, the glass continues to cool as it progresses through the die and therefore continues to contract to some extent. As a result the rigid tubing as it issues from the bottom of the die is a few thousandths of an inch smaller than the die bore so that a continuous leakage of air occurs about the tubing surrounded by the' lower region of the die bore. To assure that this leakage will not at the outset be such as to prevent sufficient reduction of air pressure about the tubing entering the die to attain expansion of the glass into contact with the die, a vacuum sourcemay be connected to the vacuum chamber at the exit end of the die.

An alternative, but less preferred form of the inven. tion utilizes a die surrounded by a vacuum chamber and having spiral slots formed lengthwise of its wall through which air is withdrawn to expand the glass into contact therewith during the initial stage of the passage of the molten tubing therethrough.

For a more detailed description of the invention refer ence is now made to the accompanying drawing, wherein FIG. 1 is a diagrammatic vertical sectional view taken through a drawing apparatus including a die embodying the invention, the section being taken on the axis of the glass tubing that is being drawn.

FIG. 2 is a view similar to FIG. 1, on an enlarged scale, showing the expanded tubing passing through the die.

F IG. 3 is a vertical sectional view of an alternative form of die embodying the invention.

Referring to FIG. 1 of the drawing, the structure shown includes a fragment of a forehearth 11 containing a supply body 12 of molten glass maintained therein by a suitable glass-melting tank (not shown) with which the forehearth is associated in a conventional fashion. The forehearth 11 has the usual glass-feeding bottom outlet 13 provided with an orifice ring 14. A suitably suspended tubular mandrel 15, in vertical register with the bore of ring 14, at its lower end carries a bore forming bell 13 at such a level that an annular passage is formed between the ring 14 and bell 18 through which a tubular molten stream of glass 19 is drawn, as by means of drawing rolls such as 20.

As will be seen, near the root thereof, stream 19 is closely surrounded by a plate 21 provided with an aperture which is of a diameter less than the diameter of the bore of die 22 whose entrance end is slightly spaced from the plate 21 to provide an annular passage to a vacuum chamber 23 screw threaded over the entrance end of the die and connected to suitable vacuum source via a conduit 25.

The exit end of die 22 is similarly provided with a plate 31, conveniently utilized as one wall of a vacuum chamber 33, and having an aperture corresponding to the diameter or cross sectional configuration of the bore of die 22. The vacuum chamber 33 is connectable to a vacuum source via a suitable conduit 35.'

Slidably resting on suitable brackets such as 32 is a 1 two-piece plate 41 that has an aperture of the diameter approximately that of the aperture of plate 21 and is axially split as at 36. Plate 41 is utilized as an iris to substantially seal the space between the unexpanded tubing and the passage through plate 31 at the commencement of a drawing operation and until such time that expanded tubing passes through the orifice of such plate. At such time the tubing simply laterally displaces the halves of the plate 41. For convenience in placement of the plate 21 about the tubing or for its replacement it may also be similarly axially split.

For the purpose of regulating the temperature of die 22 Patented June. 22, 1965 it is surrounded by a group of annular tubes such as tube 50 each having cooling medium discharge openings such as 51 directed toward die 22 and each connected to a cooling medium supply manifold 53 through a regulating valve such as 54.

For the purpose of describing the operation, conventionally formed molten tubing passing through die 22 is expanded into contact with the wall of the upper region of the die by creating vacuum within chamber 23, preferably aided by creating, at the same time, a vacuum in chamber 33. The expanded tubing is rapidly cooled by contact with the die and slightly contracts away from its surface at a viscosity too great for the vacuum to have any subsequent influence upon it. Such expanded tubing upon engaging the two halves of plate 41 simply pushes them aside, the plate 31 of the chamber thereafter functioning to maintain a suitable seal about the exit end of the die.

In the alternative form of the invention illustrated in FIG. 3 the die 60, which is interchangeable with die 22, has spiral slots such as 61 through its wall and has a vacuum chamber 62 formed thereabout. A tubular stream of molten glass passing through the die is caused to expand in the upper region of the die because of its low viscosity and then as it is chilled by the die shrinks slightly away therefrom and becomes too viscous to be affected by the negative pressure about it as it progresses through the die just as in the structure of FIGS. 1 and 2.

As will be understood, the invention in no way'hampers utilization of any of the facilities available in previous systems assisting in the regulation of the diameter and/ or wall thickness of tubing being drawn, but adds a very important regulatory feature. Also although in the disclosure the tubular dies are assumed to have cylindrical bores, it will be understood that the invention is equally applicable to formation of tubing of a wide variety of cross sectional configurations by simply providing dies of the desired cross sectional bore configurations. Also if desired not only may the tubing be drawn to any desired cross sectional configuration, but it is further possible to give it a different wall thickness in one or more radial areas than in others. For example, by so positioning the plate 21 that its aperture center is out of alignment with the die bore center, expansion of the glass at one side of the bore wall will be greater than on the opposite side, therefore the two opposite wall areas will be of different thicknesses; or if it is desired to make tubing of elliptical cross section and to have the wall region of the minor axis areas thinner than that in the major axis areas, this can be done by employment of a plate similar to 21 which has an aperture with a greater major to minor axis than that of the die and which will tend to flatten the molten tubing before its entry into the die.

Although the invention has been described with reference to but a few embodiments, it is to be understood that they are by way of examples and not by way of limitations.

What is claimed is:

1. The method of expanding molten glass tubing, as it is being continuously drawn, to a desired cross sectional size and configuration; which includes passing such tubing through the bore of a tubular die whose bore affords a space of a cross section and size substantially corresponding to the transverse configuration and size to which the tubing is to be expanded, supplying atmospheric pressure internally of the tubing being formed while creating negative pressure in said space to expand the tubing into contact with such die and maintaining the die at a temperature at which it chills the tubing contacted by it to cause the same to contract away from such die slightly during its continued passage therethrough.

2. A method as in claim 1 which comprises maintaining negative pressure about the tubing in such space by withdrawing air from about the tubing at one end of such space.

3. A method as in claim 2 which comprises also withdrawing air from about the tubing from the other end of such space.

4. A method as in claim 1 which comprises maintaining negative pressure about the tubing in such space by withdrawing air from about the tubing in such space through passages through the die running lengthwise and in part transversely thereabout.

5. A method as in claim 1 which comprises passing the tubing through an aperture immediately above the die of smaller diameter than the diameter of the die bore, whereby uniform expansion of the tubing throughout its perimeter is effected and a uniform wall thickness is attained thereabout.

6. A method as in claim 5 which comprises maintaining the axial center of the aperture laterally offset with respect to the axial center of the die bore whereby greater expansion of one side of the tube wall is effected than in its opposite side so that the wall areas on two opposite sides of its bore are of different thicknesses.

7. A method as in claim 1 which comprises passing the tubing through an elliptical aperture having a greater major to minor axis than the major and minor axes of an elliptical bore die whereby the expansion of the tubing in two opposite arcuate regions is the same but different from the expansion of two transversely located opposite arcuate regions thereof so that the wall thickness of the tube in two opposite arcuate regions differ from the wall thickness of the remaining two opposite arcuate regions.

References Cited by the Examiner UNITED STATES PATENTS 2,150,017 3/39 Barnard 65-187 X 2,519,375 8/50 Jargstorff et al. 18-14 2,780,835 2/57 Sherman 65-183 X 2,790,994 5/57 Cardot et a1. 65-110 X DONALL H. SYLVESTER, Primary Examiner. 

1. THE METHOD OF EXPANDING MOLTEN GLASS TUBING, AS IT IS BEING CONTINUOUSLY DRAWN, TO A DESIRED CROSS SECTIONAL SIZE AND CONFIGURATION; WHICH INCLUDES PASSING SUCH TUBING THROUGH THE BORE OF A TUBULAR DIE WHOSE BORE AFFORDS A SPACE OF A CROSS SECTION AND SIZE SUBSTANTIALLY CORRESPONDING TO THE TRANSVERSE CONFIGURATION AND SIZE TO WHICH THE TUBING IS TO BE EXPANDED, SUPPLYING ATMOSPHERIC PRESSURE INTERNALLY OF THE TUBING BEING FORMED WHILE CREATING NEGATIVE PRESSURE IN SAID SPACE TO EXPAND THE TUBING INTO CONTACT WITH SUCH DIE AND MAINTAINING THE DIE AT A TEMPERATURE AT WHICH IT CHILLS THE TUBING CONTACTED BY IT TO CAUSE THE SAME TO CONTRACT AWAY FROM SUCH DIE SLIGHTLY DURING ITS CONTINUED PASSAGE THERETHROUGH. 